Filter unbillable notifications before calculating

Notifications with a `billable_units` count of `0` wont have any effect
on the result, but including them in the query will slow down the
grouping and summing of the results because it’ll have to loop over more
rows.

app/dao/notifications_dao.py

@@ -221,6 +221,7 @@ def get_notification_billable_unit_count_per_month(service_id, year):
     ).order_by(
         NotificationHistory.created_at
     ).filter(
+        NotificationHistory.billable_units != 0,
         NotificationHistory.service_id == service_id,
         NotificationHistory.created_at >= start,
         NotificationHistory.created_at < end